This invention relates to a method and apparatus for repairing and/or sealing oil and gas wells and, more particularly, to a method and apparatus for sealing a cement sheath between the well casing and the wellbore in an oil or gas bearing formation.
The leakage of shallow gas through the casing cement used in well completion is often a problem in oil and gas wells. Such leakage is generally caused by inherent high pressures in oil and gas wells and can create environmental problems and compromise well safety. This leakage most often occurs because of cracks or other imperfections that occur in the cement that is injected into the well during well completion procedures between the well casing and the wellbore.
Techniques for preventing shallow gas leakage are disclosed in Rusch, David W. et al, xe2x80x9cUse of Pressure Activated Sealants to Cure Sources of Casing Pressurexe2x80x9d, SPE (Society of Petroleum Engineers) Paper 55996. These techniques use the application of an epoxy sealing technique. One disadvantage in using the technique taught by Rusch et al is that high pressure differentials across the source of leakage are required.
A common method in the oil industry to attempt to repair and seal leaking annular cement in an existing oil or gas well is to perform a cement xe2x80x9csqueezexe2x80x9d in the problem region. This is accomplished by first perforating the casing in the region to be repaired. A plug is then set immediately below the perforated zone and cement is pumped from the surface down the casing and forced through the perforations. This cement is intended to flow into the discontinuities in the existing cement or wellbore well in order to seal them once the cement solidifies.
However, the use of cement has disadvantages. The cement used for well sealing purposes has a relatively high viscosity which limits the penetration of the cement into discontinuities both in the well formation and in the cement previously used for sealing the well. Furthermore, cement has a partially solidified state before it finally solidifies which limits the application of pressure on the cement during the squeezing operation. Such partial solidified state limits the penetration of the cement into the formation or into the cement discontinuities where the gas leakage arises.
According to the invention, there is provided a method of squeezing a liquefied material previously in solid form through the perforated casing of an oil or gas well and into solid material surrounding said casing, said method comprising melting said material at a predetermined depth in said well with a heating tool and forcing said melted material through said perforated casing of said well and into said solid material surrounding said casing, said melted material being used for sealing faults within said solid material surrounding said well casing, said solid material being cement.